1. Field of the Invention
The present invention relates to a method for enciphering and deciphering instructions in a microcomputer, so that third persons cannot learn the software, and a microcomputer used for effecting the same.
2. Description of the Prior Art
Software is essential rfor the opreation of most data processing systems. It takes considerable time and effort to produce software, however, so software must be protected. If third persons can easily learn the software, the time and effort expended in its development would in effect have been wasted. Accordingly, means for preventing the software from being known or discovered by third persons have become necessary.
Conventionally, a single type of software, i.e., software coded in accordance with one predetermined coding format, is stored in a data processing system. The single type of software is read and decoded by a single decoder to obtain control signals for data processing. The control signals enable the data processing system to perform the desired processing. The relationship between the software and the decoded results, however, can be easily learned by third persons, because the decoding is effected in accordance with a predetermined decoding format which can be easily learned by third persons. This is especially true with microcomputers, where anyone can easily read out programs stored in its read only memory (ROM). Therefore, it is necessary to provide some countermeasure for protecting programs from being read by third persons.